Hi Ramona! Each Walt Disney World theme park has a
Baby Care Center which has lots of common baby supplies - baby food, baby cereals, juice, formula, pacifiers, diapers, wipes, powder and diaper creme, Pedialyte and various children's medicines too. In addition there are padded changing areas, nursing rooms and other places to sit out of the heat and feed your child. There's even a little tv viewing area and coloring supplies to keep older siblings entertained. It's a wonderful resource to escape the parks for a bit and take care of baby in a calm, cool environment.
As far as fresh fruit goes, there are a few locations in every park where you can buy fresh fruit. The Main Street Bakery in the Magic Kingdom has a big selection of grab 'n go items like fruit, cereals, pastries and more at breakfast and the items change throughout the day. There is a location like that in each park so just check your guidemap for "snack" locations or ask a cast member.
Also every resort will have a selection of fresh fruit in their food court area as well so you could stock up before you head to the parks for the day. If you are staying in a deluxe villa resort, the merchandise shop will have some fresh fruit in their market area along with other supplies like water, milk, bread and other snacks.
All in all, you should be covered! I hope you have a wonderful time with your family and let us know if you have any other questions! ~Suzannah